It is proposed to intervene pharmacologically in the biochemistry of cone receptors and to assess this intervention electrophysiologically. The goal is to identify enzyme systems present in cones which may be involved in excitation and/or adaptation. Suspect enzyme systems to be tested are phosphodiesterases which hydrolyze cyclic nucleotides, cyclases which synthesize cyclic nucleotides, kinases which phosphorylate proteins, and GTP phosphohydrolase which catalyzes the cleavage of GTP. Specific inhibitors, activators, cofactors or substrates of these enzymes will be applied to ascertain which modify photoreception. The chemicals will be applied by intracellular injection of suffusion to cones in the suffused, isolated, sliced preparation of the turtle retina. Cones will be impaled by double-barreled electrodes or drawn into suction electrodes under visual control using an infrared television system. The effects of the application of these chemicals will be assessed by monitoring the membrane potential and sensitivity of the cone to see which mimic changes in the level of illumination. If the agent modulates membrane potential, then it will be determined if it modulates the same ionic channels that are modulated by changes in illumination. This will be determined by comparative conductance measurements, ionic substitution measurements, and reversal potential measurements for both modulations produced by changes in illumination or by the chemical agent. If the effector modulates the sensitivity of cones, then the speed or response of the system, the light intensity-photoresponse characteristic curves, and photoresponse waveforms will be compared to those obtained when an equal shift in sensitivity is produced by changes in illumination. Similarly, the time course of sensitivity changes will be compared. When more is known of the physiology of photoreception, a better understanding of the pathology of human visual disorders, such as the inability to adjust to changes in ambient illumination or macular degenerations will be obtained.